For anyone wondering about extra costs for SPGS, I have a DD in Y10.
Books add up to around £200 a year if bought new and it is definitely possible to find secondhand copies for less money.
Sports kit - we spent about £300 initially. We have had to replace only a couple of things - swimsuit and tracksuit bottoms which ended up a little short/tight. Everything else I bought still fits and in any case they are pretty relaxed at this age about sports uniform unless playing in a team. Don't think DD is going to grow much more so I doubt I will have to buy anything else. There are things we bought that we barely used too, mainly the cotton hoodie. DD prefers the warm base layer and mid-layer on a cold day. The sports socks are super comfortable, and DD wears them loads in and out of school. She also really loves the tracksuit bottoms. She had abandoned the skort by Y8.
Trips have been inexpensive, maybe £100ish a year for compulsory stuff, probably less. I think Geographers do a compulsory residential which is more.
There are trips in the holidays which are more expensive but they are not compulsory and it will absolutely not be the case that anyone feels left out if they don't go as places are very limited. DD wants to go on a History trip next year which is £650 for a week in the hols.
Hope this is a bit helpful. Well done to everybody's DDs!